Happy Birthday, Pluto! (You’re Still Not a Planet, Though)

On February 18, 1930 a young astronomer named Clyde Tombaugh discovered a new object in the outer Solar System by examining photographic plates he had taken several weeks earlier. After a lot of discussion and suggestions, the name Pluto was selected for the new object, which was elevated to the status of the 9th planet in the Solar System.

Since 1930, we’ve learned a lot more about Pluto–and also about our Solar System. From the initial mass estimate ranking Pluto about the same as Earth, scientists have used the motion of Pluto’s largest satellite Charon to measure its mass to be about 18% of the mass of our Moon. (Mercury, by comparison, is about 5 times more massive than the Moon.) In 2003, another outer Solar System object later named Eris was discovered to be roughly the same size as Pluto, but slightly more massive. Along with the discovery of other distant objects with great names like Quaoar, Haumea, Makemake, Orcus, and Sedna, it became evident that Pluto isn’t particularly unique. In 2006, the International Astronomical Union (IAU) drafted a formal definition of what a “planet” is, which excluded Pluto (along with Eris and the other new discoveries).

A lot of people I’ve talked to seem to take it rather personally that Pluto has been “demoted”. As a cosmologist I don’t have a stake in this discussion (nobody consulted me, even! I’m offended!)—I don’t study planets or the Solar System, and as odd as it might seem, I don’t particularly care if Pluto is classified as a planet or not. But one thing is fairly certain: if Pluto is classified as a planet, there will be a lot more planets in the Solar System than the 9 we memorized in elementary school. No matter how you classify things, 9 is a number that doesn’t work: there are either 8 planets (under the current IAU definition) or a lot more. Keeping Pluto as a planet and excluding the others can only be justified by nostalgia, not science.

However, for the record I think we should teach elementary school kids how to spell “Quaoar”.


8 responses to “Happy Birthday, Pluto! (You’re Still Not a Planet, Though)”

  1. […] This post was mentioned on Twitter by Matthew Francis, Dr. Matthew Francis. Dr. Matthew Francis said: Happy Birthday, Pluto! (You're Still Not a Planet, Though) http://wp.me/p14KoQ-5T […]

  2. And how is “Quaoar” pronounced, I’d like to know.

    1. “KWA-oh-ar” or “KWA-war” are the two accepted pronunciations I’ve heard.

  3. […] is preparing to settle into orbit around the smallest planet (yes, I’m picking a fight with the Pluto lovers out there). Much of Mercury’s surface is still unmapped; Messenger will finish the job, and […]

  4. […] is the smallest planet (yes yes Pluto blah blah Pluto blah), and although it’s closer to Earth than the outer planets, in many ways it’s a lot […]

  5. […] is the farthest planet from the Sun (yes yes, Pluto blah blah): its orbit is about 30 times larger than Earth’s, so it’s not that easy to observe. […]

  6. […] this change anything about Pluto’s status? Probably not. An object doesn’t have to have moons to be a planet (or Mercury and Venus […]

  7. […] being large and similar to Uranus in most respects, was a shoo-in for planetary status, and when it was discovered in 1930, Pluto was classified as a planet; by that time so many asteroids had been found that Ceres and Vesta lost their planetary status by […]

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